It is commonly known that most countries are short of protein-rich nourishment. Malnutrition is a serious problem in the developing countries whereas the developed countries are lacking in fodder and feed proteins, which means that meat production from plant proteins is insufficient. With increasing demand for plant proteins and with rising prices, research work has had two main directions in this field. Exploitation of the existing supply can be increased by the addition of enzymes and other substances and, on the other hand, new protein sources are sought by plant breeding and by new agricultural technology. Some research has been done to increase protein utilization by heat treatment, too.
The utilization of food and feed proteins is very low, somewhere between 25% and 40% even under optimum conditions. Addition of enzymes and other substances does not lead to a significantly higher value, whereas heat treatment only indirectly leads to a higher rate of utilization. This fact can be explained by the mistaken belief that the material obtained by heat treatment contains completely denatured, easily digestible protein. This supposition may be derived from the well-known fact that proteins--except for a few thermophilic proteins--are heat sensitive in their solutions and are denatured at a relatively low temperature (around 50.degree. C.), which leads to a significant increase in their proteolytic digestibility whereas natural proteins can only very slowly be digested by proteolytic enzymes. In all probability these observations have led to the fallacy that easily digestible, denatured feed-proteins can be obtained by subjecting plant materials, such as fodder meals to dry heat treatment or steaming such as is carried out after extraction of oil seeds. The fallacy of this conclusion is shown by the fact that a wool-like regenerated fiber can be produced from the protein fraction of commercial feed-proteins, proving that these proteins maintain their property to associate which characterizes proteins only in their native state. In vitro experiments have also shown that only 50-60% of these proteins can be digested. This limited digestibility explains their low and uneconomical utilization in vivo.
The use of various leguminous plants, like soybeans, and other oilseeds has become general in nutrition all over the world. The nutritive value of these plants is high since they contain 20-40% of protein of high quality, providing a balanced amino acid supply. The drawback of these materials is that they contain different antinutritive substances, like trypsin inhibitors and antivitamin factors. Fortunately, these can usually be inactivated by heat treatment.
In the human diet, leguminous plants are used in cooked form, whereas products from oil seeds are prepared after defatting, heat treatment and drying. Fodder is usually uncooked, but oil seeds are used as defatted, extracted grist. Among the plants of high protein content soybean is processed in the greatest amounts. This process includes extraction with hexane or other solvents but no chemical treatment is used in the process. In this method the time of heat treatment and the control of humidity are of particular importance. The nutritive property of the products can be determined in different chemical, enzymatic and biological assays. In the most commonly used biological test, the so-called Protein Efficiency Ratio (PER) is determined with test animals. This value--determined with growing rats as test animals--is usually between 1.0-1.3 for raw soybeans, 1.4-1.6 for the moderately processed material and 2.2-2.4 for the completely processed material (e.g. treatment at 100.degree. C. for 15 minutes).
Our investigations have shown that the utilization of soybeans having even the highest PER value is not sufficient since the proteins obtained by the known procedures are not irreversibly denatured.
For the food industry materials of high protein content are desirable. To obtain such materials, defatted plant grits are extracted with aqueous solutions at the isoelectric pH of the constituent proteins. In another commonly used procedure, proteins are dissolved at a slightly alkaline pH and subsequently precipitated at isoelectric pH. Both methods entail a considerable loss of protein (Wilcke et al., J. Am. Oil. Chem. 56, 259-261; 1979).
Several chemical procedures are known also for the chemical treatment of feeds and fodders. The aim of these treatments is usually the production of milk substitutes for young animals. The following procedure can be mentioned as an example: soybean grist is partially hydrollyzed by exhaustive treatment with hydrogen chloride gas (Jones et al.: J. Anim. Sci. 45, 1073-1078; 1977). This treatment lowers the pH below 0.4 which, during the long exposure time (7 days) used in the cited procedure, leads to the hydrolysis of the treated material. As a result a highly soluble material is obtained containing large amounts of free amino acids and smaller peptides.
The aim of the chemical treatment is sometimes to decrease the high buffer capacity of the fodder which is deleterious to young animals. The buffer capacity can be influenced by adding strong electrolytes, such as sodium hydroxide, phosphoric or hydrochloric acid, or by adjusting the pH to 4 (Colvin B. M., Ramsey, H. A.; J. Dairy Sci. 51, 898-904; 1968).